"Most people have noticed that when a speaker of any size is placed against a wall, it’ll deliver stronger bass response. This can be a strong temptation to use this acoustic phenomenon (a function of standing waves) to provide some additional heft, especially from smaller (less than 8”) woofers. But this room-enhanced bass will have an uneven frequency ...
One of the best resources that deals with this subject is to be found on the site of Genelec, speaker manufacturer.
Specifically, page 9 of this document is worth a read.
That's caused by reflections off the walls interfering with the direct sound from the speaker. At some frequencies, the reflection will cancel out the direct sound, at other frequencies they will add up, so the frequency response changes from the intended one.
The manual may contain some information on the intended placement of the speakers.